Formats, like packages and subroutines, are declared rather than executed, so they may occur at any point in your program. (Usually it's best to keep them all together though.) They have their own namespace apart from all the other ``types'' in Perl. This means that if you have a function named ``Foo'', it is not the same thing as having a format named ``Foo''. However, the default name for the format associated with a given filehandle is the same as the name of the filehandle. Thus, the default format for STDOUT is name ``STDOUT'', and the default format for filehandle TEMP is name ``TEMP''. They just look the same. They aren't.
Output record formats are declared as follows:
format NAME =
FORMLIST
.
If name is omitted, format ``STDOUT'' is defined. FORMLIST consists of
a sequence of lines, each of which may be one of three types:
Picture lines are printed exactly as they look, except for certain fields that substitute values into the line. Each field in a picture line starts with either ``@'' (at) or ``^'' (caret). These lines do not undergo any kind of variable interpolation. The at field (not to be confused with the array marker @) is the normal kind of field; the other kind, caret fields, are used to do rudimentary multi-line text block filling. The length of the field is supplied by padding out the field with multiple ``<'', ``>'', or ``|'' characters to specify, respectively, left justification, right justification, or centering. If the variable would exceed the width specified, it is truncated.
As an alternate form of right justification, you may also use ``#'' characters (with an optional ``.") to specify a numeric field. This way you can line up the decimal points. If any value supplied for these fields contains a newline, only the text up to the newline is printed. Finally, the special field ``@*'' can be used for printing multi-line, nontruncated values; it should appear by itself on a line.
The values are specified on the following line in the same order as the picture fields. The expressions providing the values should be separated by commas. The expressions are all evaluated in a list context before the line is processed, so a single list expression could produce multiple list elements. The expressions may be spread out to more than one line if enclosed in braces. If so, the opening brace must be the first token on the first line. If an expression evaluates to a number with a decimal part, and if the corresponding picture specifies that the decimal part should appear in the output (that is, any picture except multiple ``#'' characters without an embedded ``."), the character used for the decimal point is always determined by the current LC_NUMERIC locale. This means that, if, for example, the run-time environment happens to specify a German locale, ``,'' will be used instead of the default ``.''. See the perllocale manpage and the section on WARNINGS for more information.
Picture fields that begin with ^ rather than @ are treated specially. With a # field, the field is blanked out if the value is undefined. For other field types, the caret enables a kind of fill mode. Instead of an arbitrary expression, the value supplied must be a scalar variable name that contains a text string. Perl puts as much text as it can into the field, and then chops off the front of the string so that the next time the variable is referenced, more of the text can be printed. (Yes, this means that the variable itself is altered during execution of the write() call, and is not returned.) Normally you would use a sequence of fields in a vertical stack to print out a block of text. You might wish to end the final field with the text ``...'', which will appear in the output if the text was too long to appear in its entirety. You can change which characters are legal to break on by changing the variable $: (that's $FORMAT_LINE_BREAK_CHARACTERS if you're using the English module) to a list of the desired characters.
Using caret fields can produce variable length records. If the text to be formatted is short, you can suppress blank lines by putting a ``~'' (tilde) character anywhere in the line. The tilde will be translated to a space upon output. If you put a second tilde contiguous to the first, the line will be repeated until all the fields on the line are exhausted. (If you use a field of the at variety, the expression you supply had better not give the same value every time forever!)
Top-of-form processing is by default handled by a format with the same name as the current filehandle with ``_TOP'' concatenated to it. It's triggered at the top of each page. See the write entry in the perlfunc manpage.
Examples:
# a report on the /etc/passwd file
format STDOUT_TOP =
Passwd File
Name Login Office Uid Gid Home
------------------------------------------------------------------
.
format STDOUT =
@<<<<<<<<<<<<<<<<<< @||||||| @<<<<<<@>>>> @>>>> @<<<<<<<<<<<<<<<<<
$name, $login, $office,$uid,$gid, $home
.
# a report from a bug report form
format STDOUT_TOP =
Bug Reports
@<<<<<<<<<<<<<<<<<<<<<<< @||| @>>>>>>>>>>>>>>>>>>>>>>>
$system, $%, $date
------------------------------------------------------------------
.
format STDOUT =
Subject: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$subject
Index: @<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$index, $description
Priority: @<<<<<<<<<< Date: @<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$priority, $date, $description
From: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$from, $description
Assigned to: @<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$programmer, $description
~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$description
~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$description
~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$description
~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$description
~ ^<<<<<<<<<<<<<<<<<<<<<<<...
$description
.
It is possible to intermix print()s with write()s on the same output
channel, but you'll have to handle $- ($FORMAT_LINES_LEFT)
yourself.
select((select(OUTF),
$~ = "My_Other_Format",
$^ = "My_Top_Format"
)[0]);
Pretty ugly, eh? It's a common idiom though, so don't be too surprised
when you see it. You can at least use a temporary variable to hold
the previous filehandle: (this is a much better approach in general,
because not only does legibility improve, you now have intermediary
stage in the expression to single-step the debugger through):
$ofh = select(OUTF);
$~ = "My_Other_Format";
$^ = "My_Top_Format";
select($ofh);
If you use the English module, you can even read the variable names:
use English;
$ofh = select(OUTF);
$FORMAT_NAME = "My_Other_Format";
$FORMAT_TOP_NAME = "My_Top_Format";
select($ofh);
But you still have those funny select()s. So just use the FileHandle
module. Now, you can access these special variables using lowercase
method names instead:
use FileHandle;
format_name OUTF "My_Other_Format";
format_top_name OUTF "My_Top_Format";
Much better!
format Ident =
@<<<<<<<<<<<<<<<
&commify($n)
.
To get a real at or caret into the field, do this:
format Ident =
I have an @ here.
"@"
.
To center a whole line of text, do something like this:
format Ident =
@|||||||||||||||||||||||||||||||||||||||||||||||
"Some text line"
.
There is no builtin way to say ``float this to the right hand side
of the page, however wide it is.'' You have to specify where it goes.
The truly desperate can generate their own format on the fly, based
on the current number of columns, and then eval() it:
$format = "format STDOUT = \n";
. '^' . '<' x $cols . "\n";
. '$entry' . "\n";
. "\t^" . "<" x ($cols-8) . "~~\n";
. '$entry' . "\n";
. ".\n";
print $format if $Debugging;
eval $format;
die $@ if $@;
Which would generate a format looking something like this:
format STDOUT =
^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$entry
^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<~~
$entry
.
Here's a little program that's somewhat like fmt(1):
format = ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ~~ $_
.
$/ = '';
while (<>) {
s/\s*\n\s*/ /g;
write;
}
Here's one strategy: If you have a fixed-size footer, you can get footers by checking $FORMAT_LINES_LEFT before each write() and print the footer yourself if necessary.
Here's another strategy: Open a pipe to yourself, using open(MYSELF, "|-") (see the open() entry in the perlfunc manpage) and always write() to MYSELF instead of STDOUT. Have your child process massage its STDIN to rearrange headers and footers however you like. Not very convenient, but doable.
For example:
$str = formline <<'END', 1,2,3;
@<<< @||| @>>>
END
print "Wow, I just stored `$^A' in the accumulator!\n";Or to make an swrite() subroutine which is to write() what sprintf() is to printf(), do this:
use Carp;
sub swrite {
croak "usage: swrite PICTURE ARGS" unless @_;
my $format = shift;
$^A = "";
formline($format,@_);
return $^A;
}
$string = swrite(<<'END', 1, 2, 3);
Check me out
@<<< @||| @>>>
END
print $string;
Lexical variables (declared with ``my") are not visible within a format unless the format is declared within the scope of the lexical variable. (They weren't visible at all before version 5.001.)
Formats are the only part of Perl which unconditionally use information from a program's locale; if a program's environment specifies an LC_NUMERIC locale, it is always used to specify the decimal point character in formatted output. Perl ignores all other aspects of locale handling unless the use locale pragma is in effect. Formatted output cannot be controlled by use locale because the pragma is tied to the block structure of the program, and, for historical reasons, formats exist outside that block structure. See the perllocale manpage for further discussion of locale handling.